Uncovering the mechanisms of oxide scale growth and scale/metal interface degradation of a NiCoCrAlYHf alloy in air-50 %H2O at 1000 °C

This work presents a comprehensive study on the mechanisms of oxide scale growth and scale/metal interface degradation of a NiCoCrAlYHf alloy in air-50 %H₂O at 1000 °C. Compared to air oxidation, water vapor prevents the formation of spinel but initiates the growth of γ-Al₂O₃ in the early oxidation stage. During prolonged oxidation in air-50 %H₂O, γ-Al₂O₃ sustains continuous growth and undergoes a slow transformation to α-Al₂O₃. Apart from the selective oxidation of Al, the embedded metals into the oxide scale due to enhanced scale/metal interface migration in air-50 %H₂O results in the generation of γ/α-Al₂O₃ alloyed with base metal cations, such as Ni^2 + , Co²⁺ and Cr³⁺. The stabilization of γ-Al₂O₃, and the low vacancy formation energies of aluminum and oxygen due to the interstitial protons originating from H₂O dissociation and the substitution of base metal cations onto Al³⁺ sites in the γ/α-Al₂O₃ contribute to fast oxide growth in air-50 %H₂O. Additionally, the interface imperfections composed of large interface pores and oxide intrusions resulting from accelerated Al diffusion and the excessive consumption of reactive elements (RE) in the form of RE-rich clusters aggravate the scale/metal interface degradation. Ultimately, both the accelerated oxide growth and the scale/metal interface degradation trigger premature spallation failure of oxide scale in air-50 %H₂O. Our findings provide new insights into the oxidation and failure mechanisms of NiCoCrAlYHf alloys under high-water-vapor conditions at high temperatures.